If you're not looking at "All Time" data, and you've actually configured a custom multi-year time period, but you're not seeing data older than 26 months or so, then you're probably being impacted by Google Analytics new default retention period.

In May/June of 2018, Google notified all Google Analytics' users that they would be archiving and deleting any data older than 26 months unless users specifically opted in to have their data retained longer (or indefinitely).

If you didn't see that message, and/or didn't choose a longer data retention time, then any data older than 26 months has been deleted by Google.

This has nothing to do with AgencyAnalytics; you'll see the same thing if you log into your account at Google.com/Analytics.

AgencyAnalytics is only able to display the data that currently exists within your native Google Analytics account. If the data doesn't exist in your GA account, we can't display it in our platform either.